A variety of fluid flow control valves and corresponding valve actuators are utilized for on/off control or throttling the flow of fluid, such as in a gas or oil pipeline system, or in other process fluid systems. The fluid flow control valves are typically sliding stem control valves or rotary action control valves and are operated by a valve actuator such as a pneumatic piston or diaphragm actuator responding to the output of a valve positioner or valve controller instrument for accurate throttling control of the valve.
In the case of rotary action control valves, these units typically employ a flow control member in the form of a rotatable ball, rotatable ball segment, or a rotatable butterfly element. The rotation of the flow control element opens and closes the valve gate or plug.
Valve actuators for controlling such rotary action control valves typically employ a linearly movable member, such as a movable diaphragm connected to a rod at the diaphragm center. Moving the diaphragm displaces the rod linearly and thus requires a linear to rotary action translation. A rotational lever arm of a rotatable lever has one end fixed to the valve rotary shaft and the other lever arm end is coupled to the diaphragm rod. Linear movement of the diaphragm rod moves the lever arm, rotates the lever and thereby actuates a rotational movement in the valve shaft which is ultimately connected to the rotatable flow control element in the fluid control valve.
As noted previously, rotary actuators typically change linear motion to rotary motion using a ridged connection from the moving diaphragm rod to a rotational lever. Referring to FIG. 1(a) there is illustrated existing linkage in a rotary valve actuator wherein the lever arm 200 defined between the center of the rotatable lever 202 and the linkage point 204 with the actuator diaphragm rod 206 is at 45.degree. to the centerline of the actuator diaphragm rod 206. The resultant torque lever arm (shown as 0.7071 reference length) is defined between the center of the lever 202 and the centerline of the actuator diaphragm rod.
As the diaphragm rod 206 pushes on the lever arm 200 in response to linear actuating movement of a movable actuator member 208, the lever arm rotates the lever 202 through 90.degree. rotation. When the lever arm is again at 45.degree. to the linkage centerline, as in FIG. 1(c), the torque lever arm at a reference length of 0.7071 is 30% shorter than when the torque lever arm is a reference length of 1.0 at 90.degree. to the centerline as in FIG. 1(b).
Therefore, as the lever is rotated from the beginning of actuator travel as shown in FIG. 1(a), the actuator torque resulting from the torque lever arm and the driving linear actuator force is low. At the mid-travel position shown in FIG. 1(b), the actuator torque is high, and at the end of the travel position shown in FIG. 1(c), the actuator torque is again low. The typical rotary actuator therefore provides a low-high-low actuator torque output to the associated rotary valve during valve actuation.
However, during valve actuation most rotary valves require a high torque at the beginning of travel to rotate the valve disc, for instance, out of the valve seat. High torque also is required to seat the valve disc at the end of the travel. In other words a high-low-high actuator torque output is desired rather than the currently available low-high-low actuator torque output.